一：I2C設備操作方式：
1. 應用程序操作法：i2c的設備的驅動可以直接利用linux內核提供的i2c-dev.c文件提供的ioctl函數接口在應用層實現對i2c設備的讀寫，但是在應用層使用ioctl函數對應用程序員要求較高，需要自行構建msg結構體，必須了解設備的操作流程，時序之類的。
這方式實現需要用用程序員調用 read， write， ioctl， open， close等linux標准文件接口操作/dev/i2c(X)設備文件。
2. 驅動程序操作法：i2c設備的驅動也可以通過普通的設備驅動實現，像往常的驅動一樣實現，然后在應用層就可以像讀取普通文件一樣操作，無需再考慮讀寫時序。其實普通的設備驅動也可以用兩種方法實現，
1）構建字符設備驅動，在open,read,write等函數中直接操作i2c總線的相關寄存器來讀寫i2c設備，但是這種方法因平台不同，設備不同都要重新寫驅動
2）在設備驅動中調用i2c-core.c提供的i2c_transfer函數來實現和i2c設備的通信，這樣只要對不同的設備寫不同的驅動就行了。
二：首先我們來看一看應用程序操作法：
標准i2c接口提供的標准的設備/dev/i2c，首先我們看看linux下標准i2c的結構。
在/linux-kernel-3.8/drivers/i2c目錄下
----Algos/ 一些i2c總線適配器通信的算法，個人感覺是用I/O口模擬實現i2c通信的算法
----Busses/ I2C總線驅動的方法，對應於君正4775適配器驅動的文件是i2c-jz4775.c
----Chips/ I2C設備驅動，具體到某個設備，比如at24c08等
----I2c-boardinfo.c 定義i2c_register_board_info 注冊i2c設備相關信息（其實就是將要注冊的i2c設備添加到i2c設備鏈表）
----I2c-core.c I2C核心文件，用於聯系設備驅動和總線驅動，作為一個橋梁，有用的函數i2c_add_addapter
i2c_add_driver，和i2c_transfer函數 （其實就是定義了一些標准的API接口函數）
----I2c-dev.c 通用的i2c設備驅動（其實就是生成的標准的/dev/i2c字符設備文件接口）
----Kconfig
----Makefile
我們要選擇編譯/linux-kernel-3.8/drivers/i2c/相關的platform driver和arch/mips/xburst/soc-4775/board//trunk/core/core-misc.c相結合最終生成的固件就會有/dev/i2c0標准接口。
以下是筆者做qn8025FM驅動時寫下的通過標准i2c設備文件實現的讀寫函數：通過這兩個讀寫函數就可以直接讀寫i2c設備的寄存器實現想用的功能
/*
* Version: V1.0.2.
* Date: 2014-11-25.
*/
#include "qn8025_i2c.h"
/*源頭即是 IOCTL 呼叫, 通道為 I2C_SMBUS,
而 args 則定義了: 讀或寫, 位置, 長度, 以及回傳值的資訊內容.*/
//ioctl(file, I2C_SMBUS, (i2c_smbus_ioctl_data*)msgset);
static inline __s32 i2c_smbus_access(int file, char read_write, __u8 command,
int size, union i2c_smbus_data *data)
{
struct i2c_smbus_ioctl_data args;
args.read_write = read_write;
args.command = command;
args.size = size;
args.data = data;
int ret= ioctl(file,I2C_SMBUS,&args);//set Smbus transfer
//printf("ioctl-read_i2c: %#x\n", args.data->byte);
return ret;
}
//向設備發送一個比特
static inline __s32 i2c_smbus_write_quick(int file, __u8 value)
{
return i2c_smbus_access(file,value,0,I2C_SMBUS_QUICK,(union i2c_smbus_data *)0);
}
static inline __s32 i2c_smbus_read_byte(int file)
{
union i2c_smbus_data data;
if (i2c_smbus_access(file,I2C_SMBUS_READ,0,I2C_SMBUS_BYTE,&data))
return -1;
else
return 0x0FF & data.byte;
}
static inline __s32 i2c_smbus_write_byte(int file, __u8 value)
{
return i2c_smbus_access(file,I2C_SMBUS_WRITE,value,
I2C_SMBUS_BYTE,(union i2c_smbus_data *)0);
}
static inline __s32 i2c_smbus_read_byte_data(int file, __u8 command)
{
union i2c_smbus_data data;
data.byte = 0;
// printf("before read_byte: %#x\n", data.byte);
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
I2C_SMBUS_BYTE_DATA,&data))
return -1;
else{
// printf("read_byte: %#x\n", data.byte);
return 0x0FF & data.byte;
}
}
static inline __s32 i2c_smbus_write_byte_data(int file, __u8 command,
__u8 value)
{
union i2c_smbus_data data;
data.byte = value;
// printf("before write_byte: %#x\n", data.byte);
int ret = i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_BYTE_DATA, &data);
// printf("write_byte: %#x\n", data.byte);
return ret;
}
static inline __s32 i2c_smbus_read_word_data(int file, __u8 command)
{
union i2c_smbus_data data;
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
I2C_SMBUS_WORD_DATA,&data))
return -1;
else
return 0x0FFFF & data.word;
}
static inline __s32 i2c_smbus_write_word_data(int file, __u8 command,
__u16 value)
{
union i2c_smbus_data data;
data.word = value;
return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_WORD_DATA, &data);
}
static inline __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value)
{
union i2c_smbus_data data;
data.word = value;
if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_PROC_CALL,&data))
return -1;
else
return 0x0FFFF & data.word;
}
/* Returns the number of read bytes */
static inline __s32 i2c_smbus_read_block_data(int file, __u8 command,
__u8 *values)
{
union i2c_smbus_data data;
int i;
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
I2C_SMBUS_BLOCK_DATA,&data))
return -1;
else {
for (i = 1; i <= data.block[0]; i++)
values[i-1] = data.block[i];
return data.block[0];
}
}
static inline __s32 i2c_smbus_write_block_data(int file, __u8 command,
__u8 length, const __u8 *values)
{
union i2c_smbus_data data;
int i;
if (length > 32)
length = 32;
for (i = 1; i <= length; i++)
data.block[i] = values[i-1];
data.block[0] = length;
return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_BLOCK_DATA, &data);
}
/* Returns the number of read bytes */
/* Until kernel 2.6.22, the length is hardcoded to 32 bytes. If you
ask for less than 32 bytes, your code will only work with kernels
2.6.23 and later. */
static inline __s32 i2c_smbus_read_i2c_block_data(int file, __u8 command,
__u8 length, __u8 *values)
{
union i2c_smbus_data data;
int i;
if (length > 32)
length = 32;
data.block[0] = length;
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
length == 32 ? I2C_SMBUS_I2C_BLOCK_BROKEN :
I2C_SMBUS_I2C_BLOCK_DATA,&data))
return -1;
else {
for (i = 1; i <= data.block[0]; i++)
values[i-1] = data.block[i];
return data.block[0];
}
}
static inline __s32 i2c_smbus_write_i2c_block_data(int file, __u8 command,
__u8 length,
const __u8 *values)
{
union i2c_smbus_data data;
int i;
if (length > 32)
length = 32;
for (i = 1; i <= length; i++)
data.block[i] = values[i-1];
data.block[0] = length;
return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_I2C_BLOCK_BROKEN, &data);
}
/* Returns the number of read bytes */
static inline __s32 i2c_smbus_block_process_call(int file, __u8 command,
__u8 length, __u8 *values)
{
union i2c_smbus_data data;
int i;
if (length > 32)
length = 32;
for (i = 1; i <= length; i++)
data.block[i] = values[i-1];
data.block[0] = length;
if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_BLOCK_PROC_CALL,&data))
return -1;
else {
for (i = 1; i <= data.block[0]; i++)
values[i-1] = data.block[i];
return data.block[0];
}
}
//檢測i2c設備功能
/*static int check_funcs(int file, int size, int daddress, int pec)
{
unsigned long funcs;
//check adapter functionality
if (ioctl(file, I2C_FUNCS, &funcs) < 0) {//獲取i2c設備支持的功能
fprintf(stderr, "Error: Could not get the adapter "
"functionality matrix: %s\n", strerror(errno));
return -1;
}
switch (size) {
case I2C_SMBUS_BYTE:
if (!(funcs & I2C_FUNC_SMBUS_READ_BYTE)) {
fprintf(stderr, MISSING_FUNC_FMT, "SMBus receive byte");
return -1;
}
if (daddress >= 0
&& !(funcs & I2C_FUNC_SMBUS_WRITE_BYTE)) {
fprintf(stderr, MISSING_FUNC_FMT, "SMBus send byte");
return -1;
}
break;
case I2C_SMBUS_BYTE_DATA:
if (!(funcs & I2C_FUNC_SMBUS_READ_BYTE_DATA)) {
fprintf(stderr, MISSING_FUNC_FMT, "SMBus read byte");
return -1;
}
break;
case I2C_SMBUS_WORD_DATA:
if (!(funcs & I2C_FUNC_SMBUS_READ_WORD_DATA)) {
fprintf(stderr, MISSING_FUNC_FMT, "SMBus read word");
return -1;
}
break;
}
if (pec
&& !(funcs & (I2C_FUNC_SMBUS_PEC | I2C_FUNC_I2C))) {
fprintf(stderr, "Warning: Adapter does "
"not seem to support PEC\n");
}
return 0;
}
*/
//設置i2c從設備地址
static int set_slave_addr(int file, int address, int force)
{
/* With force, let the user read from/write to the registers
even when a driver is also running */
if (ioctl(file, force ? I2C_SLAVE_FORCE : I2C_SLAVE, address) < 0) {
fprintf(stderr,
"Error: Could not set address to 0x%02x: %s\n",
address, strerror(errno));
return -errno;
}
return 0;
}
//打開i2c設備
static int open_i2c_dev(int i2cbus, char *filename, size_t size, int quiet)
{
int file;
snprintf(filename, size, "/dev/i2c/%d", i2cbus);
filename[size - 1] = '\0';
file = open(filename, O_RDWR);//打開i2c設備
if (file < 0 && (errno == ENOENT || errno == ENOTDIR)) {
sprintf(filename, "/dev/i2c-%d", i2cbus);
file = open(filename, O_RDWR);
}
if (file < 0 && !quiet) {
if (errno == ENOENT) {
fprintf(stderr, "Error: Could not open file "
"`/dev/i2c-%d' or `/dev/i2c/%d': %s\n",
i2cbus, i2cbus, strerror(ENOENT));
} else {
fprintf(stderr, "Error: Could not open file "
"`%s': %s\n", filename, strerror(errno));
if (errno == EACCES)
fprintf(stderr, "Run as root?\n");
}
}
return file;
}
//向i2c設備寫數據 (0, 0x58, , 9300, 0)
static int i2c_write_data(int i2cbus,int addr,int daddr,int set_val,int vmask)
{
int res, file;
int value;
char filename[20];
//打開設備描述符，可以自動尋找合適的設備名
file = open_i2c_dev(i2cbus, filename, sizeof(filename), 0);
if (file < 0 || set_slave_addr(file, addr, 1)) {
// perror("open i2c");
printf("open %s fail\n", filename);
return -1;
}
//掩碼，可以單獨設置某一位而不影響其他位
if (vmask) {
int oldvalue;
oldvalue = i2c_smbus_read_byte_data(file, daddr);
printf("write and read\n");
if (oldvalue < 0) {
fprintf(stderr, "Error: Failed to read old value\n");
close(file);
return -1;
}
value = (set_val & vmask) | (oldvalue & ~vmask);
} else {
value = set_val;
}
res = i2c_smbus_write_byte_data(file, daddr, value);
if (res < 0) {
fprintf(stderr, "Error: Write failed\n");
close(file);
return -1;
}
//res = i2c_smbus_read_byte_data(file, daddr);
close(file);
//if (res < 0) {
// printf("Warning - readback failed\n");
//} else
//if (res != value) {
// printf("Warning - data mismatch - wrote 0x%0*x, read back 0x%0*x\n",2,value,2, res);
//} else {
//printf("Value 0x%0*x written, readback matched\n",2, value);
return 0;//讀到的值和寫入的值一致，才返回成功
//}
//return -1;
}
//從i2c讀取數據
static int i2c_read_data(int i2cbus,int addr,int daddr,unsigned char *read_data)
{
int file,res;
char filename[20] = {0};
//打開i2c設備
file = open_i2c_dev(0, filename, sizeof(filename), 0);
if (file < 0 || set_slave_addr(file, addr, 1))//設置從設備地址
return -1;
res = i2c_smbus_read_byte_data(file, daddr);
close(file);
if (res < 0) {
fprintf(stderr, "Error: Read failed\n");
return -1;
}
*read_data = res;
//printf("read data -- 0x%x\n",*read_data);
return 0;
}
/* qn8025 set "set_value" to register "daddr"
addr: slave address
daddr: register address
set_val: the value wiil put into register
vmask: value maske
*/
int qn8025_i2c_write_data(int addr, int daddr, int set_val, int vmask)
{
return i2c_write_data(0, addr, daddr, set_val, vmask);
}
/* qn8025 get "read_data" from register "daddr"
addr: slave address
daddr: register address
set_val: the value wiil get from register
*/
int qn8025_i2c_read_data(int addr, int daddr, unsigned char *read_data)
{
return i2c_read_data(0, addr, daddr, read_data);
}